Electroless coating of cobalt and nickel



United States Patent 3,472,665 ELECTROLESS COATING OF COBALT AND NICKELElton D. Prueter, Saginaw, and Wilhelm E. Walles, Midland,-Mich.,assignors to The Dow Chemical Company,

Midland, Mich., a corporation of Delaware No Drawing.Continuation-impart of application Ser. No. 590,516, Oct. 31, 1966,which is a continuation-in-part of application Ser. No. 297,959, July26, 1963. This application June 2, 1967, Ser. No. 643,059

Int. Cl. C09d /10; C03c 25/02 US. Cl. 106-1 16 Claims ABSTRACT OF THEDISCLOSURE A continuous film of cobalt or nickel can be applied to solidmetallic and nonmetallic surfaces by immersion in a solution of a cobaltor nickel salt in a liquid 2-oxazolidinone at a temperature above about200 C.

This application is a continuation-in-part of US. patent applicationSer. No. 590,516, filed Oct. 31, 1966, and now abandoned, which was in trn a continuation-in-part of US. Patent application Ser. No. 297,959,filed July 26, 1963, and now abandoned.

BACKGROUND STATEMENT OF THE INVENTION A new process has been discoveredfor depositing a continuous cobalt or nickel film on a solid substrateby contacting. (a) the solid substrate with a solution of (b) a cobaltor nickel salt and (c) a liquid 2-oxazolidinone at a temperature aboveabout 200 C. for a time sufiicient to deposit a continuous cobalt ornickel film thereupon. More specifically, the electroless coating bathis a solution of about 0.01-50 weight percent of cobalt or nickel saltin a liquid 2-oxazolidinone of the formula:

where each R individually is H, C -C alkyl or C -C hydroxyalkyl andwhere together the R groups contain not more than 9 carbons. Preferably,it is a solution of about l-2-0 weight percent of the cobalt or nickelsalt in S-methyl-Z-oxazolidinone heated at a temperature of about2'00'-300 C. The resulting cobalt or nickel coating on a clean substrateis a very uniform, continuous and tightly adhering film.

The process is elegant in its simplicity. It avoids difficult hightemperature and vacuum techniques as well as complex chemical orelectrolytic reduction systems. The process is highly specific forcoating with cobalt or nickel. Yet, at the same time, it is applicableto .a wide variety of metallic and nonmetallic solid substrates. Forexample, applied to a non-conductor, it gives effective surface elec-3,472,665 Patented Oct. 14, 1969 trical conductivity. Applied to anetched magnesium plate, it provides a superior surface for chromeplating. It is particularly suitable for thin coatings, e.g., about01-50 IIllCI'OIlS.

GENERAL DESCRIPTION Substrates to be coated with cobalt or nickel bythis process must be solid and essentially inert under the processconditions. Porous materials which swell in contact with the platingbath are usually poor substrates. In general rigid, non-porous materialsare most suitable. Thermoplastic materials can be used provided thesoftening point is at least 10 C. higher than the required bathtemperature.

A great variety of metallic and nonmetallic substrates can be used.Thin, shiny layers of cobalt or nickel can be applied to such metals asgold, platinum, silver, copper, lead, tin, antimony, bismuth, magnesiumand aluminum as well as to such alloys as bronze, brass and steel. Notethat the process will deposit cobalt or nickel on a metal higher in theelectrochemical series. Suitable inorganic substrates include asbestos,glass, titanium phosphate, titanium dioxide, silicon carbide, alumina,carbon black, calcium carbonate, calcium metasilicate, calcium silicate,kaolin, talc, silica, diatomaceous earth, quartz and the like incrystalline or amorphous form. Other electrically nonconductivesubstrates include solid thermoset resinous polymers such asmelamine-formaldehyde resins, phenolfurfural resin, polyester and epoxyresins, silicone resins as well as irradiated polyethylene andpolypropylene. Filled hydraulic cements such as transite and filledphenolformaldehyde or urea-formaldehyde resin also can be used.

The actual shape of the substrate is not critical so long as the surfaceto be coated can be contacted with the 2- oxazolidinone bath. Theprocess can be used with substrates which are regular or irregular,crystalline or amorphous, a film, foil, fiber or wire, etc. It isparticularly suited for mirror coating glass and metallic surfaces andfor coating inorganic crystal needles or whiskers.

Cobalt and nickel salts suitable for use in this process should have asolubility of at least 0.01 weight percent in the 2-oxazolidinone bathand preferably are in essentially anhydrous form. Inorganic mineral acidsalts such as cobaltous chloride, bromide and iodide, nickel chloride,bromide and iodide, cobaltous sulfate, nickel sulfate and the like areoperable. However, salts of weak organic acids such as cobaltic andcobaltous acetylacetonate, nickel acetylacetonate, cobaltous acetate,nickel acetate, cobaltous formate, nickel formate, cobaltous propionateand nickel propionate are preferred, particularly cobalt and nickelsalts of C C carboxylic acids.

The third critical element is the liquid 2-oxazolidinone which serves asa reaction medium and also is intimately involved in providing theessential cobalt or nickel atoms for the coating process. Suitable2-oxazolidinones can be made, for example, by the process of Walles, US.Patent 3,179,667. These liquids have the general formula:

where each R individually is H, C -C alkyl or C -C hydroxyalkyl andwhere together the R groups contain not more than 9 carbons. Typical ofthe hydroxyalkyl -C H (OH) and -C H (OH) Among such solvents5-methyl-2-oxazolidinone is preferred.

In practice a solution containing 0.01-50 weight percent, preferablyabout 1-20 weight percent, of the cobalt or nickel salt in the2-oxazolidinone is normally used as the coating bath. Although minoramounts of water can be tolerated, the system is preferably essentiallyanhydrous because of the operating temperature above about 200 C. Acoating temperature of about 200-300 C. and preferably about 225 275 C.is normally used. At lower temperatures deposition of cobalt or nickelis too slow. To obtain maximum coating adhesion, the substrate surfaceshould be clean and dry before immersion in the coating bath. Eflicientagitation of the bath is required to assure optimum surface contact.

The cobalt or nickel coating is deposited very uniformly. Its thicknessis a function primarily of the contact time, bath temperature andreagent concentration. Since the coating continues as long as cobalt ornickel atoms are provided, there is no real limit on the coatingthickness. But in practice the process is best suited for thin 0.1-50micron coatings. Typically a 0.5-5.0 micron nickel coating can beapplied in 1-2 hours using a l0% solution of nickel acetylacetonate at230260 C.

The following examples further illustrate this invention. Unlessotherwise indicated, all parts and percentages are by weight.

Example l.Cobalt films on glass (A) A mixture of 0.78 g. (2.0 mmoles) ofcobaltic acetylacetonate and 20 g. (200 mmoles) of 5-methyl-2-oxazolidinone charged to a thoroughly cleaned glass test tube and thenheated to 250 C. After 1.5 hr. at 250 C., the solution was removed and acontinuous cobalt film was found on the inside wall of the test tube.

(B) Similar experiments were carried out using 1 and 2 percent solutionsof various cobalt salts and 2-oxazolidinones as shown in Table 1. Ineach case a continuous coating of cobalt was deposited on the inner wallof the test tube.

TABLE 1 2-oxazolidi- Run Cobalt salt none Conditions 113-1- cobaltousacetate.-. 5-methyl. 250 0., 2 hrs. 113-2 Cobaltous formate do. 250 0.,2 hrs. 113-3 Cobaltous acetate. 5 ethyl. 5 0. 113-4 Cobaltous chloride5-methy 113-5- Cobaltic acetylacetonate do Example 2.Nickel films onglass (A) A nickel coating bath was prepared by dissolving 0.26 parts ofnickel acetylacetonate in parts of 5- methyl-Z-oxazolidinDne. A nickelmirror formed on a clean glass surface immersed for 3 hrs. in this bathat 250-255 C. However, the nickel mirror was not so highly adherent tothe glass as a cobalt mirror made concurrently using a bath containing0.40 parts of cobaltic acetylacetonate in 10 parts of5-methyl-2-oxazolidone.

(B) The procedure of Example 2A was repeated with a test tube rigorouslycleaned with bichromate/sulfuric acid, rinsed with water and finallyisopropanol. A better adhering Ni-mirror was obtained.

Example 3.Electroconductive cobalt coatings (A) A piece of commercialTransite asbestos-filled cement building panel A thick was coated withcobalt by immersion in a solution of 5 parts cobaltic acetylacetonate in100 parts 5-methyl-2-oxazolidinone for 2.5 hrs. at 245 -250 C. Thecoated Transite strip had a grayish metallic appearance with a shinymetallic luster in some places.

The surface electrical resistance of the Transite panel was measured onseveral sides with an ohm meter and surface electrodes mounted 4.5 cm.apart with the following results:

Ohms Original Transite 20 l0 Co coated Transite 8-28 The surfaceresistance of the coated Transite is low enough for electrolyticdeposition of other metals on top of the cobalt.

(B) The procedure of Example 2A was repeated with a plastic bottle capmade of a urea-formaldehyde resin filled with carbon black. A graycobalt coating on the cap was obtained having the following surfaceresistance between electrodes 0.8 cm. apart:

Ohms Original resin 20X10 Co coated resin 10-30 The films of cobalt andnickel deposited on glass and other inert substrates by the processes ofthis invention are not affected by acetone, water nor prolonged exposureto air. Mirrors thereof do not transmit but do reflect light. They areparticularly useful in reflecting special wavelengths of light.

Example 4.Cobalt coatings on metal surfaces To explore the deposition ofcobalt on metal surfaces, a solution of 8 parts cobaltic acetylacetonatein 100 parts 5-methyl-2-oxazolidinone was used as a standard coatingbath with a contact time of 2 hrs. at 245'-265 C. The surfaces of thetest pieces were cleaned as necessary with steel wool and then wipedwith a soft paper. After treatment the pieces were washed thoroughly,dried and the coating thickness measured by X-ray fluorescence. Typicalresults are given in Table 2.

TABLE 2.COBAL'1 COATING OF VARIOUS METALS Coat thickness Base metal(micron) Remarks 0. Shiny.

1. 6 Do. 1. 3 Shiny and dark areas. Lead (foil) 0. 40 Black. Copper(strip) 0. Shiny. Brass (rod) 1. 5 Do. Aluminum (strip). 0. G8 Dull.Antimony (chunk)- 0. 01 Tarnishcd.

Example 5.-Nickel coatings on metal surfaces The general procedure ofExample 4 was followed using a solution of 6 parts nickelacetylacetonate in parts of 5-methyl-2-oxazolidinone with a contact timeof 55 min. at 234235 C. Typical results are given in Table 3.

TABLE 3.NICKEL COATING OF "ARIOUS METALS 1 Tln (M.P. 232 C.) was coatedat 220 C. for 2 hrs.

Similar attempts to deposit iron, chromium, zinc and aluminum on copperfrom a solution of the corresponding acetylacetonate in5-methyl-2-oxazolidinone were unsuccessful even at a temperature closeto the HP. of the 5-methyl-2-oxazolidinone, about 270 C.

Example 6.Nickel coatings on inorganic crystals (A) About 140 parts ofS-methyl-Z-oxazolidinone was heated under a nitrogen atmosphere to about115 C. Then 15.0 parts of nickel acetylacetonate and 15.0 parts oftitanium phosphate crystal needles were added. The transparent titaniumphosphate crystals had the composition 2TiO 'P O and were about 15-30microns in length and 1-3 microns in thickness. The well stirred mixturewas heated for 1 hour at 235255 C. Then, the crystals were recovered byfiltration and washed by slurrying with water and then with acetone togive 17.0 parts of dry black crystalline powder. The product contained14.9% Ni, 27% Ti and 17% P. The presence of nickel in elementary formwas confirmed by X-ray analysis. Microscopic examination clearly revealsa veryuniform, nontransparent nickel coating, without visible openspots.

(B) In another experiment an extremely thin coating of nickel wasapplied to the titanium phosphate by reducing the contact time. Theresulting coated crystals contained only 2.1% Ni.

(C) In a similar manner nickel coatings have also been deposited oncrystalline needles of titanium dioxide and barium titanate. The processis also applicable to crystals of silicon carbide, carbon and alumina,for example, and to coating with cobalt as well as nickel.

We claim:

1. A method for depositing a continuous film of cobalt or nickel on asolid substrate which comprises contacting (a) the solid substrate witha nonaqueous solution of (b) about 0.01-50 weight percent of a cobalt ornickel salt and (c) a liquid 2-oxazolidinone of the formula:

where each R individually is H, C -C alkyl or C -C hydroxyalkyl andwhere together the R groups contain not more than 9 carbons, at atemperature above about 200 C. for a time sufficient to deposit acontinuous cobalt or nickel film thereon.

2. The process of claim 1 where the solid substrate is a metal.

3. The process of claim 1 where the solid substrate is an inorganiccrystal.

4. The process of claim 1 where the solid substrate is a nonconductivesolid resin.

5. The method of claim 1 where the cobalt or nickel salt is a salt of amineral acid.

6. The method of claim 1 where the cobalt or nickel salt is the salt ofa C C carboxylic acid.

7. The method of claim 1 where the salt is cobalt or nickelacetylacetonate.

8. The method of claim 1 where the 2-oxazolidinone isS-methyl-Z-oxazolidinone.

9. The method of claim 1 wherein the deposition temperature is about 200 300 C.

10. A solution for an electroless coating of substrates with cobalt ornickel comprising about 0.0lweight percent of a cobalt or nickel salt ina liquid 2-oxazolidinone of the formula:

where each R individually is H, C -C alkyl or C -C hydroxyalky-l andwhere together the R groups contain not more than 9 carbons.

11. The solution of claim 10 where the salt is an acetylacetonate andthe liquid is 5-methyl-2-oxazolidinone.

12. A solid substrate coated with a continuous film of cobalt or nickelby the process of claim 1.

13. The product of claim 12 where the substrate is ametal.

14. The product of claim 12 where the substrate is an inorganic crystal.

15. The product of claim 12 where the substrate is a nonconductive solidresin.

16. The product of claim 12 where the coating has a thickness of about0.1-50 microns.

References Cited UNITED STATES PATENTS 3,250,784 5/ 1966 Gensheimer eta1. 3,294,578 12/1966 Popeck.

JULIUS FROME, Primary Examiner L. HAYES, Assistant Examiner U.S. C1.X.R.

